In recent years, there have been advances in the development of nonaqueous electrolyte secondary batteries with higher energy densities as progress toward a reduction in the size and weight of electric appliances. Furthermore, the expansion of fields of application of nonaqueous electrolyte secondary batteries require further improvement in battery characteristics.
Hitherto, metallic lithium, metal compounds, such as elemental metals, oxides, and alloys of lithium, capable of storing and releasing lithium, and carbonaceous materials have been used as negative-electrode active materials for use in nonaqueous electrolyte secondary batteries. With respect to carbonaceous materials, in particular, for example, nonaqueous electrolyte secondary batteries including carbonaceous materials, such as coke, artificial graphite, and natural graphite, capable of storing and releasing lithium have been reported. In such nonaqueous electrolyte secondary batteries, lithium does not exist in the metallic state, suppressing the formation of dendrites and improving the life and safety of batteries. In particular, nonaqueous electrolyte secondary batteries including graphite-based carbonaceous materials such as artificial and natural graphite have been receiving attention because they should meet the demand for higher capacity.
In nonaqueous electrolyte secondary batteries including the carbonaceous materials, usually, cyclic carbonic esters, such as propylene carbonate and ethylene carbonate, are widely used as high-dielectric solvents for use in nonaqueous electrolytic solutions. In particular, for nonaqueous electrolyte secondary batteries including non-graphite-based carbonaceous materials, e.g., coke, propylene carbonate-containing solvents are suitably used.
In the case of using solvents containing propylene carbonate in nonaqueous electrolyte secondary batteries including negative electrodes composed of graphite-based carbonaceous materials alone or mixtures of graphite-based carbonaceous materials and other negative-electrode materials capable of storing and releasing lithium, however, the decomposition reaction of propylene carbonate proceeds vigorously on surfaces of the electrodes during charging. This makes it impossible to smoothly store and release lithium at the graphite-based carbonaceous negative electrodes.
In contrast, ethylene carbonate is not vigorously decomposed; hence, in nonaqueous electrolyte secondary batteries including graphite-based carbonaceous negative electrodes, ethylene carbonate is often used as a high-dielectric solvent in electrolytic solutions. Even in the case of using ethylene carbonate as a main solvent, however, there are problems of reductions in charge and discharge efficiency and cycle characteristics, an increase in internal battery pressure due to gas generated by the decomposition of electrolytic solutions on surfaces of the electrodes during charging and discharging, and the like.
To improve characteristics of nonaqueous electrolyte secondary batteries, electrolytic solutions containing various additives have been reported.
To suppress the decomposition of electrolytic solutions of nonaqueous electrolyte batteries including graphite-based negative electrodes, for example, the following electrolytic solutions have been reported: electrolytic solutions containing vinylene carbonate and its derivatives (Patent Document 1), electrolytic solutions containing ethylene carbonate derivatives having non-conjugated unsaturated bonds in their side chains (Patent Document 2), and electrolytic solutions containing halogen atom-substituted cyclic carbonates (Patent Document 3). These compounds contained in the electrolytic solutions are reductively decomposed on surfaces of negative electrodes to form films that suppress excessive decomposition of the electrolytic solutions.
These compounds, however, do not necessarily meet the requirements for storage characteristics under a high-temperature environment, battery characteristics in a high-voltage state, or gas generation. Vinylene carbonate compounds react readily with positive electrode materials in a charged state. A higher vinylene carbonate compound content is liable to cause a further reduction in storage characteristics.
Meanwhile, in place of vinylene carbonate and its derivatives and ethylene carbonates having nonconjugated unsaturated bonds in their side chains and their derivatives, nitrile compounds having unsaturated bonds have been reported as additives capable of being reductively decomposed to form films (Patent Document 4). The patent document discloses that also in electrolytic solutions containing these compounds, the reductive decomposition of solvents can be suppressed at a low level during charging. Also in the case of using these compounds, however, there are still issues of battery characteristics under a high-temperature environment and high-voltage conditions or gas generation.
The suppression of the reactivity of electrode materials in a charged state to solvents, vinylene carbonate and its derivatives, ethylene carbonates having nonconjugated unsaturated bonds in their side chains and their derivatives, or halogen atom-substituted cyclic carbonates, which are additives, results in improvements in battery characteristics under high-temperature conditions and the suppression of gas generation. It is desirable to develop a technique for suppressing the reactivity.
In place of propylene carbonate and ethylene carbonate, a halogen atom-substituted cyclic carbonate used as a high-dielectric solvent has been reported (Patent Document 5). The document describes that the incorporation of a fluorine atom or a chlorine atom serving as an electron-withdrawing group in ethylene carbonate suppresses the decomposition and improves charge and discharge efficiency. This effect, however, is still insufficient under a high-temperature environment. Thus, further improvements are required.
In recent years, negative-electrode active materials composed of elemental metals, such as silicon (Si), tin (Sn), and lead (Pb), capable of being alloyed with lithium, alloys containing at least these metal elements, and metal compounds containing these metal elements (hereinafter, referred to as “negative-electrode active materials containing Si, Sn, Pb, and the like”) have been reported. The capacity of these materials are about 2,000 mAh·cm−3 or more and is about four or more times those of graphite and the like. Thus, the use of these materials results in a higher capacity.
Secondary batteries including the negative-electrode active materials containing Si, Sn, Pb, and the like can have higher capacities but have problems of a reduction in safety, a reduction in charge and discharge efficiency due to the deterioration of the negative-electrode active materials by charge and discharge, a reduction in battery characteristics under a high-temperature environment and high-voltage conditions, gas generation, and a reduction in cycle characteristics.
To ensure safety and prevent a reduction in the discharge capacity of such batteries, a nonaqueous electrolytic solution containing a phosphotriester and a cyclic carbonate or a multimer of a carbonate has been reported as a nonaqueous electrolytic solution used for secondary batteries (Patent Document 6). To improve charge-discharge cycle characteristics of batteries, a method for improving charge-discharge cycle characteristics of a battery by adding a heterocyclic compound having a sulfur atom and/or an oxygen atom in its ring to a nonaqueous electrolytic solution and forming a film on a surface of a negative-electrode active material has been reported (Patent Document 7). To suppress gas generation when a battery is stored in a charged state at a high temperature, a negative electrode provided with a fired mixture layer containing negative-electrode active material particles, lithium oxide, and a binder arranged on a surface of a current collector has been reported (Patent Document 8).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 8-45545
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2000-40526
[Patent Document 3] WO98/15024
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2003-86247
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 62-290072
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 11-176470
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 2004-87284
[Patent Document 8] Japanese Unexamined Patent Application Publication No. 2007-66726